For efficient manufacturing of filters it is known to insert a pleated or folded filter matrix into an injection molded plastic frame. Glue is then filled into the space between the filter matrix and the frame to fix and seal the filter matrix in the frame. The glue is then allowed to harden. This may take some time and either means a slowing down of the manufacturing process or requires cost increasing intermediate storage before the filter is packed or subjected to additional working steps. The glue must have precisely the right viscosity when administrated to the space between the filter matrix and the filter frame. This means that without proper and costly apparatus a great many filters are rejected because of leaks and they may also at times lack in appearance with glue here and there. There exists therefore for this type of filter a need for an improved way to seal the filter matrix in the frame.
In accordance with the invention the above object is solved by the following steps:
a) A preformed pleated filter matrix is provided in which the plead ends are sealed, e.g. by continuous applied glue strings (or by dipping the ends in glue).
b) Then, the filter matrix is placed in a frame.
c) A tool is held against said filter and the frame.
d) Finally, a heated sealing thermoplastic material is injected under pressure into a space between the filter matrix, frame and tool. The volume of the injected material is less than the actual volume of the space between filter matrix and frame, so that the entire space is never entirely filled.
The injected thermoplastic material will in the same process step be shaped by the tool and form a seal between filter matrix and frame.
Surprisingly enough the expected total collapse of the unsupported filter matrix does not occur. The reason for this is presumably that although the pressure of the entering thermoplastic material is very high the pressure at the propagation front of the injected material will be approximately the same as the surrounding atmospheric pressure. From the injection point the pressure will thus fall rapidly, preventing the total pressure on the filter from crushing the pleats, which would inevitably occur if a remaining free space was not left unfilled.
The high pressure used with injection molding technique makes it possible with a fast filling process and since the space between frame and filter matrix is comparatively narrow the thermal content of the sealing material will rapidly be taken by the surrounding, frame, filter material and air so that also the hardening will be fast. Since the filter pleats are sealed in their ends it is not necessary for the seal between the matrix and the frame to cover the entire ends of the pleats, and it is instead sufficient if only a continuous seal is established all around the filter matrix. Since the seal material is only visible on the top of the filter the local amount or how far deep the sealant extends does not influence the appearance of the filter. The pressure of the sealant when injected is still sufficient to result in a good filling out of the tool so that a very controlled appearance of the ready filter can be achieved.
If too much material tends to agglomerate at some location the propagation front will there advance down into the space preventing a build up of too high pressures and forces on the pleats. However since the space normally becomes more narrow downwards the flow resistance will increase inducing an even distribution around the filter. The ends of the filter pleats will also function to this end since the pleats taper downwards.
In particular in the case of large filters, previously it sometimes was necessary to provide several sealant inlets into the space between the matrix and the frame in order not to reach too high pressures locally. This is undesired since every inlet is comparatively expensive and there are many different filters meaning high tooling costs. Another feature and advantage of the present invention uses the filter frame itself or the filter frame in combination with a molding tool or some other part of the filter to form a distribution channel for the sealant around the filter. In order to increase the pressure and thus the transport in the channel without increasing the pressure and thus forces on the filter matrix holes are provided in a wall or flange of the filter through which the sealant may flow.
Alternatively a protective layer on the filter matrix that extends laterally out from the matrix may have openings allowing only a restricted flow into the sealing area. The openings or passages may be of different size and distribution to promote an even distribution all around the filter. This may be used to save tooling costs. The protective layer also may serve as a transport protection that is removed when the filter is used, or may be permanent and air permeable.
Normally a filter is then completed in a subsequent step by the application of a gasket. This gasket may be of foam that is applied on top of a lateral flange or groove a nozzle following the contour of the filter. By careful control of viscosity nozzle size, speed etc the gasket can be given comparatively accurate dimensions. This way of fabricating the gasket is however due to the many requirements troublesome and frequently results in adjustment work and rejection which is most undesired. In accordance with a further feature of the invention it is also possible to achieve this gasket in a simple, reliable and fast manner.
This is achieved by a manufacturing step in which the filter matrix is sealed relative the frame and the surrounding gasket for the frame molded by the tool. An elastic thermoplastic material is used for the gasket and the seal between filter matrix and frame.
The channel in the tool or between the tool forming the gasket can advantageously be used also to distribute the sealing material around the filter to give a more uniform distribution before the sealant enters into the space between matrix and frame. Also possible holes in the frame become hidden between gasket and seal and at the same time they can serve to xe2x80x9crivetxe2x80x9d the gasket to the frame. By using holes or passages from the gasket xe2x80x9cchannelxe2x80x9d in the tool the elastic material is forced to fill out the gasket before entering into the seal space, which will therefore occur comparatively uniform around the filter.